The biggest difference between new energy vehicles and traditional cars is
that they are driven by batteries, so power batteries are the core of new energy
vehicles. The power output of electric vehicles relies on batteries, and the
battery management system BMS (Battery Management System) is the core. It is a
system that monitors and manages the battery. It controls the battery by
collecting and calculating parameters such as voltage, current, temperature, and
SOC. The charge and discharge process, the management system that protects the
battery and improves the comprehensive performance of the battery, is an
important link between the vehicle power battery and the electric vehicle.
Foreign companies that are relatively good at BMS include UMC, Continental,
Delphi, AVL and FEV, etc. Now they basically follow the AUTOSAR architecture and
ISO26262 functional safety requirements. The software has more functions and has
higher reliability and accuracy. . Many domestic OEMs also have self-developed
BMS products and applications. In the early stage, their functions and
performance were far from those of first-class foreign companies. However, with
the rapid development of domestic battery and BMS technology, the gap is
gradually narrowing. It is hoped that in the near future Achieve success to
catch up or even surpass. BMS mainly consists of three parts: hardware,
underlying software and application layer software.
Hardware 1. Architecture The topology structure of BMS hardware is divided
into two types: centralized and distributed: (1) Centralized type is to
concentrate all electrical components into a large board. The sampling chip
channel has the highest utilization and the sampling chip is connected to the
main chip. Daisy chain communication can be used between them. The circuit
design is relatively simple and the product cost is greatly reduced. However,
all acquisition wire harnesses will be connected to the motherboard, which poses
greater challenges to the security of the BMS. There may also be problems with
the stability of daisy chain communication. question. It is more suitable for
situations where the battery pack capacity is relatively small and the module
and battery pack types are relatively fixed. (2) Distribution includes a main
board and a slave board. One battery module may be equipped with a slave board.
The disadvantage of such a design is that if the number of cells in the battery
module is less than 12, it will cause a waste of sampling channels (generally
there are 12 sampling chips channel), or 2-3 slave boards collect all battery
modules. This structure has multiple sampling chips in one slave board. The
advantages are high channel utilization, cost saving, system configuration
flexibility, and adaptability to different capacities and Modules and battery
packs of different specifications and types.
2. The design and specific selection of functional hardware should be
combined with the functional requirements of the entire vehicle and battery
system. Common functions mainly include collection functions (such as voltage,
current, and temperature collection), charging port detection (CC and CC2) and
charging wake-up ( CP and A+), relay control and status diagnosis, insulation
detection, high-voltage interlocking, collision detection, CAN communication and
data storage and other requirements.
(1) The main controller processes the information reported by the slave
controller and the high-voltage controller, and at the same time judges and
controls the operating status of the power battery based on the reported
information, implements BMS-related control strategies, and makes corresponding
fault diagnosis and processing. (2) The high-voltage controller collects and
reports the total voltage and current information of the power battery in real
time, realizes on-time integration through its hardware circuit, and provides
accurate data for the main board to calculate the state of charge (State of
Charge, SOC) and health state (State of Health, SOH). At the same time Precharge
detection and insulation detection functions can be realized.
(3) The controller collects and reports power battery cell voltage and
temperature information in real time, and feeds back the SOH and SOC of each
string of cells. It also has a passive balancing function, effectively ensuring
the consistency of the cells during power use.
(4) The sampling control wiring harness provides hardware support for
various information collection of power batteries and information interaction
between controllers. At the same time, a redundant insurance function is added
to each voltage sampling line to effectively avoid external short circuits of
the battery caused by wiring harnesses or management systems. .
3. Communication method There are two ways of transmitting information
between the sampling chip and the main chip: CAN communication and daisy chain
communication. CAN communication is the most stable, but due to the need to
consider the high cost of power supply chips, isolation circuits, etc., daisy
chain communication is actually It is SPI communication. The cost is very low
and the stability is relatively poor. However, with the increasing pressure on
cost control, many manufacturers are changing to the daisy chain method.
Generally, two or more daisy chains will be used to enhance it. Communication
stability.
4. Structure BMS hardware includes power IC, CPU, sampling IC, high drive
IC, other IC components, isolation transformer, RTC, EEPROM and CAN module, etc.
The CPU is the core component, and Infineon's TC series is generally used.
Different models have different functions, and the configuration of the AUTOSAR
architecture is also different. Sampling IC manufacturers mainly include Linear
Technology, Maxim, Texas Instruments, etc., including collecting cell voltage,
module temperature, and peripheral configuration balancing circuits. The
underlying software is divided into many common functional modules according to
the AUTOSAR architecture, reducing dependence on hardware and enabling
configuration of different hardware, while the application layer software
changes little. The application layer and the bottom layer need to determine the
RTE interface, and consider DEM (Fault Diagnosis Event Management), DCM (Fault
Diagnosis Communication Management), FIM (Function Information Management) and
CAN communication reserved interfaces from the aspect of flexibility, and
configure them by the application layer . The application layer software
software architecture mainly includes high and low voltage management, charging
management, state estimation, balance control and fault management, etc.
1. High and low voltage management Generally, when the power is turned on
normally, the VCU will wake up the BMS through the 12V of the hard wire or CAN
signal. After the BMS completes the self-test and enters standby, the VCU sends
a high-voltage command, and the BMS controls the closing relay to complete the
high-voltage. When powering off, VCU sends a high voltage command and then
disconnects to wake up 12V. When the gun is plugged in and charging in the
power-off state, it can be woken up by the CP or A+ signal.
2. Charging management (1) Slow charging: Slow charging uses an AC charging
pile (or 220V power supply) to convert AC into DC through a car charger to
charge the battery. Charging pile specifications generally include 16A, 32A and
64A, or through household power supply. to charge. The BMS can be woken up
through CC or CP signals, but it should be ensured that it can sleep normally
after charging. The AC charging process is relatively simple and can be
developed in accordance with the detailed regulations of the national standard.
(2) Fast charging: Fast charging uses DC charging piles to output DC to charge
the battery, which can achieve 1C or higher rate charging. Generally, 80% of the
battery can be charged in 45 minutes. It is awakened by the A+ signal of the
auxiliary power supply of the charging pile. The fast charging process in the
national standard is relatively complicated. There are two versions of 2011 and
2015. Moreover, charging pile manufacturers have different understandings of the
technical details of the national standard process that are not clear, which
also affects the vehicle charging adaptability. It is a huge challenge, so fast
charging adaptability is a key indicator to measure the performance of BMS
products.
3. Estimation function (1) SOP (StateOfPower) mainly obtains the current
available charging and discharging power of the battery through temperature and
SOC lookup tables. The VCU determines how to use the current vehicle based on
the sent power value. It is necessary to consider both releasing battery
capacity and protecting battery performance, such as limiting part of the power
before reaching the cut-off voltage. Of course, this will have a certain impact
on the driving experience of the entire vehicle. (2) SOH (StateOfHealth) mainly
represents the current health status of the battery, which is a value between
0-100%. It is generally believed that the battery cannot be used after it is
lower than 80%. It can be expressed by changes in battery capacity or internal
resistance. When using capacity, the actual capacity of the current battery is
estimated through the battery operating process data. The ratio to the rated
capacity is SOH. Accurate SOH will improve the accuracy of other modules'
estimation of battery degradation. (3) SOC (StateOfCharge) belongs to the BMS
core control algorithm and represents the current remaining capacity state. It
mainly uses the ampere-hour integration method and the EKF (Extended Kalman
Filter) algorithm, combined with correction strategies (such as open circuit
voltage correction, full correction, charging End correction, capacity
correction under different temperatures and SOH, etc.). The ampere-hour
integration method is relatively reliable under the condition of ensuring the
accuracy of current collection, but it is not robust. Due to the error
accumulation, it must be combined with a correction strategy. The EKF is more
robust, but the algorithm is more complex and difficult to implement. Domestic
mainstream manufacturers can generally achieve an accuracy within 6% at room
temperature, but estimation at high and low temperatures and battery decay is
difficult. (4) The SOE (StateOfEnergy) algorithm is currently not developed by
many domestic manufacturers, or a relatively simple algorithm is used to look up
the table to obtain the ratio of the remaining energy to the maximum available
energy in the current state. This function is mainly used for remaining cruising
range estimation.
4. Fault diagnosis is divided into different fault levels according to
different performance conditions of the battery. Under different fault levels,
BMS and VCU will take different processing measures, such as warning, power
limit or direct cut off of high voltage. Faults include data acquisition and
rationality faults, electrical faults (sensors and actuators), communication
faults, and battery status faults.
5. Balance control The balance function is to eliminate the inconsistency
of battery cells produced during battery use. According to the short board
effect of the barrel, the cell with the worst performance reaches the cut-off
condition first during charging and discharging, and the other cells There are
still some capabilities that have not been released, causing battery waste.
Balance includes active balance and passive balance. Active balance is the
transfer of energy from more monomers to less monomers, which does not cause
energy loss. However, the structure is complex, the cost is high, and the
requirements for electrical components are also high. Relatively speaking,
passive balance is The balancing structure is simple and the cost is much lower,
but the energy will be dissipated and wasted in the form of heat. Generally, the
maximum balancing current is about 100mA. Nowadays, many domestic manufacturers
use passive balancing to achieve better balancing effects. Conclusion The BMS
control method, as the central control idea of power batteries, directly affects
the service life of power batteries and the safe operation and vehicle
performance of electric vehicles. It has a significant impact on battery life
and determines the future of new energy vehicles. A good battery management
system will greatly promote the development of new energy vehicles.
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